Batteries with high capacity or high energy density represented by lithium ion secondary batteries are increasingly needed for various applications. However, if excessive current flows through such batteries, the battery temperature may rise abnormally. Development is under way for batteries that can provide improved safety in the event of such abnormality.
Batteries typically include a positive electrode, a negative electrode, a separator interposed therebetween, and an electrolyte. The separator has a porous structure because it is required in normal use to have ion conductivity as well as to electrically insulate the positive electrode from the negative electrode. The separator is sometimes required to soften and close the pores therein in the event of abnormality as mentioned above in the battery, thereby to lose its ion conductivity and stop the battery reaction (hereinafter referred to as a “shutdown property”). Separators made of polyolefins such as polyethylene and polypropylene are usually used as the separator having the shutdown property.
However, if the battery temperature rises even after the shutdown, meltdown occurs, that is, the separator melts down, and consequently, the positive and negative electrodes are short-circuited. It is preferable, therefore, to use a separator having good balance between the shutdown property and the resistance to meltdown. Patent Literature 1 focuses on separators made of polypropylene which has comparatively high melting point and good thermal stability among polyolefins. For example, a separator having a multilayer structure in which the outermost layers are a polypropylene layer and the intermediate layer(s) is a polyethylene layer is proposed.
Further, in response to the need for batteries with higher capacity, batteries including a wound electrode group are widely used. The wound electrode group is formed by spirally winding a belt-like positive electrode, a belt-like negative electrode, and a pair of separators insulating these from each other, using a winding core. Winding is usually performed by sandwiching the end portions of the pair of separators in a winding core, and winding with the positive electrode and the negative electrode being alternately interposed between the separators. Upon completion of winding, the sandwiching force around the portion of the separator from which winding is started is loosened, to remove the winding core.
However, if the slippage property of the separator surface is poor, the friction between the surface of the separator and the winding core is increased, and as a result, when removing the winding core, the separator is pulled together with the winding core or damaged. The displacement or damage of the separator leads to product defects involving current leakage.
In order to suppress the displacement of the separator, Patent Literature 2 proposes that a resin layer including polyethylene or fluorocarbon resin, the resin layer having good slippage property, be provided on the separator at a portion from which winding is started. Patent Literature 3 proposes that polypropylene containing at least 50 ppm of calcium stearate be used in the outer layer of a porous separator having a multilayer structure.